The present invention discloses a device for fracturing rock and stimulating production in an oil or gas well using pulses of pressure released within the confines of a wellbore. A specific method is presented for generating multiple pulses of pressure in succession at one location using multiple electric arc discharges. It is well-known that pulses of pressure can produce increased fracture branching and fracture density, but fractures tend to be relatively short in prior methods. The present invention adds the step of applying a static background pressure to hold open the fractures so that pulses can be applied multiple times to lengthen the fractures and increase the reach into the formation. The use of an electric arc device enables this added step. Greater surface exposure of hydrocarbon-bearing rock in the target formation results, which, in turn, leads to more complete drainage of hydrocarbons. In addition, the fractures can be better confined within the formation resulting in reduced fluid filling of the fractures relative to the hydrocarbons produced.
To illustrate the problem and the technical need, FIG. 1 shows data on the extent of hydraulic fracturing in the vertical direction for nearly four hundred hydraulic fracture treatments in the Marcellus shale in the Appalachian Basin of Ohio, Pennsylvania, and West Virginia. Many of the hydraulic fractures extend 1000 ft. in the vertical direction, with a few fractures extending nearly 2000 ft. A similar trend is found in hydraulic treatments in the Barnet Shale in the Fort Worth Basin in Texas, with average vertical fracture lengths somewhat less than those produced in the Marcellus shale.
While hydraulic fracture lengths up to 2,000 feet long are generated in the Marcellus, the Marcellus stratum itself is less than 100 ft. thick over most of West Virginia, Ohio, and Western Pennsylvania. In this circumstance, most of the fracture volume extends far outside of the production zone into unproductive strata, resulting in inefficient use of hydraulic fluids. In addition, wide spaces exist between fractures resulting in incomplete drainage of hydrocarbons in the target formation. There is a need for an improved stimulation method that maintains more fractures closer to the production zone where increased fracture surface in the target formation will emit more gas from the rock strata and less hydraulic fluid will be required for a given production level. Reduced fracturing fluid volumes will improve process efficiencies and simplify environmental management of flowback fluids at the surface of the well. Better fracture containment closer to the production layer will also help ease public concerns that long uncontrolled hydraulic fractures in conventional methods that may inadvertently intersect abandoned wells or natural faults in rock layers above the Marcellus, compromising the thick rock barriers that normally prevent the vertical migration of hydraulic fracturing fluids into drinking water aquifers. Higher fracture density in the target formation will improve the drainage of hydrocarbons from the formation.
In related prior art, Gas Propellant Fracturing (GPF) has been shown to generate increased fracture density using pulses of pressure and the method is sometimes used to stimulate wells. The GPF technique is occasionally referred to by various other names including Tailored Gas Pulse Fracturing, Controlled Pulse Fracturing, and Low-Explosive Well Stimulation. GPF applies a large impulsive force to the rock formation around the wellbore. In GPF methods, impulsive forces are produced by relatively slow-burning chemical mixtures that are considered deflagrants rather than explosives. Typical burn times are on the order of tens of milliseconds. Suitable burn times and pressure profiles must be maintained in order to fracture rock effectively. Pressures applied too quickly compress the rock, making it less permeable. Pressures applied too slowly lead to bi-wing fractures that generate limited flow to the wellbore. Intermediate impulse periods produce the best stimulation results with fractures in many directions, not just in vertical planes perpendicular to the direction of least compression in the rock formation. More extensive fracture branching is produced when properly tailored pressure pulses are applied, resulting in increased flow and increased resource recovery near the wellbore.
The pulsed electric discharges in the present invention provide an electromagnetic improvement of the GPF detonations. Unlike GPF methods, the proposed electric-discharge method can provide precise electronic control of the all-important pulse-duration and pulse-profile, yielding optimal fracturing under variable conditions in the formation. Discharge-induced pressures can be programmed on a pulse-by-pulse basis to accommodate highly variable rock properties near the wellbore.
Another major limitation of GPF methods is that the deflagrant is consumed after each shot, so that GPF devices must be replaced between shots. In contrast, electric pulses powered from the surface of the well as taught in the present invention can be fired repeatedly without removing the arc-source from the well. The arc-source can remain in the wellbore until the entire well has been treated and it can be fired repeatedly at one location. In firing multiple discharges at one location, static or quasi-static bias pressure helps to hold open the fracture channels so that superimposed transient pressure surges applied repeatedly at one location in the wellbore can better expand the fracture network and extend fractures greater distances from the wellbore than GPF methods, which already produce fractures up to about 50 feet from the wellbore in a single discharge.
To help understand the basic premise that pulsed pressures will induce highly branched fractures, a circuit analog of a pulsed hydraulic fracturing scenario is provided in FIG. 3. The analog is based upon correspondences that can be drawn between fluid parameters for pulsed hydraulic fracturing and electrical parameters for a discrete-element circuit. These correspondences are tabulated in Table I.
TABLE IElectrical parameters and analogous fluid parameters in the circuit analogused to help understand how increased fracture branching can occurin response to pulsedELECTRICALCIRCUITANALOGOUS FLUID PARAMETERVoltagePressureElectrical CurrentFluid Flow RateOhmic ResistanceFlow Resistance to Static PressureInductanceFluid and Rock Inertia
The circuit example contains three circuit branches coming off of the main circuit branch, analogous to three small fluid-filled fractures extending from various points along a principal fracture. The overall length of the principal fracture is denoted by the parameter L in the figure caption. In this example, resistance-per-unit-length of the side branches is a factor of ten higher than the resistance-per-unit-length of the principal branch, modeling a situation in which the side fractures have relatively high resistance to fluid flow due to smaller initial cross sections compared to the principal fracture.
FIG. 4 shows the voltage differences, analogous to pressure differentials, across the various branches of the circuit. The colors of the curves in FIG. 4 correspond to the colors of probe pairs in FIG. 3. Each pair of probes measures a voltage difference between the probe locations. FIG. 4 indicates that the highest voltage difference and, by analogy, the highest pressure differential occurs in the side branch nearest the transient source that drives the system. Transient voltages, and therefore transient pressures, fall off away from the drive source with very little drive remaining at the end of the principal branch. Interpreting these results in terms of analogous fluid parameters, the substantially higher “pressures” developed near the “pressure source” at the root of the “principal fracture” will tend to create multiple fractures near the root of principal fracture in preference to extending the length of the principal fracture which has much lower transient “pressure” at its extremity, illustrating the general premise of the invention.